Abstract. The remote sensing of the atmospheric greenhouse gases methane (CH4) and
carbon dioxide (CO2) in the troposphere from instrumentation aboard
satellites is a new area of research.
In this manuscript, results obtained from
observations of the up-welling radiation in the near-infrared by SCIAMACHY
on board ENVISAT
are presented.
Vertical columns of CH4, CO2 and
oxygen (O2) have been retrieved and the (air or) O2-normalised CH4 and
CO2 column amounts, the dry air column
averaged mixing ratios XCH4 and XCO2
derived.
In this manuscript the first results, obtained by using the version 0.4 of the
Weighting Function Modified (WFM) DOAS retrieval algorithm applied to SCIAMACHY
data, are described and compared with global models.
For the set of individual cloud free measurements over land
the standard deviation of the
difference with respect to the models is in the range ~100–200 ppbv
(5–10%) for XCH4 and ~14–32 ppmv
(4–9%) for XCO2.
The inter-hemispheric difference of the methane mixing ratio,
as determined from single day data,
is in the range
30–110 ppbv and in reasonable agreement with the corresponding model data (48–71 ppbv).
The weak inter-hemispheric difference of the CO2 mixing ratio can also be detected
with single day data.
The spatiotemporal pattern of the measured and the modelled XCO2
are in reasonable agreement. However,
the amplitude of the difference between the maximum and the
minimum for SCIAMACHY XCO2 is about ±20 ppmv which is about a factor of four larger than
the variability of the model data which is about ±5 ppmv.
More studies are needed to explain the observed differences.
The XCO2 model field shows low CO2 concentrations beginning of January 2003 over a
spatially extended CO2 sink region located in southern tropical/sub-tropical
Africa. The SCIAMACHY data also show low CO2 mixing ratios
over this area.
According to the model the sink region becomes a source region
about six months
later and exhibits higher mixing ratios. The SCIAMACHY and the model data
over this region show a
similar time dependence over the period from January
to October 2003.
These results indicate that for the first time a regional
CO2 surface source/sink region has been detected by measurements from space.
The interpretation of the SCIAMACHY CO2 and CH4 measurements is
difficult, e.g., because the error analysis of the currently implemented retrieval
algorithm indicates that the retrieval errors are on the same order as the small
greenhouse gas mixing ratio changes that are to be detected.